usb: usb251xb: Document chip variant usb2422

The next patch introduces support for the USB2422. Add it to the list of
devices supported by the binding.
Signed-off-by: NUwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: NRob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191023105250.16537-2-u.kleine-koenig@pengutronix.deSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/usb251xb.txt

@@ -7,7 +7,7 @@ Required properties :
  - compatible : Should be "microchip,usb251xb" or one of the specific types:
 	"microchip,usb2512b", "microchip,usb2512bi", "microchip,usb2513b",
 	"microchip,usb2513bi", "microchip,usb2514b", "microchip,usb2514bi",
-	"microchip,usb2517", "microchip,usb2517i"
+	"microchip,usb2517", "microchip,usb2517i", "microchip,usb2422"
  - reg : I2C address on the selected bus (default is <0x2C>)
 
 Optional properties :